1. Field of the Invention
The present invention relates to the bleaching of cotton griege goods, cotton blends and selected other cellulosic materials and, more particularly, to methods of treating the materials prior to bleaching to enhance the resulting bleached cloth whiteness or to reduce the bleaching time or intensity.
2. Description of the Prior Art
The chemical bleaching of cotton goods is, of course, very well known and extensively practiced in various forms using a miscellany of chemical bleaches and additives thereto, and employing various types and forms of batch and continuous bleaching equipment. The purpose of these processes is, generally, to achieve a minimum preselected whiteness level for the bleached goods in as economical and environmentally safe a manner as is possible. To this end, most conventional bleaching processes known to the art employ sodium hypochloride, hydrogen peroxide, peracetic acid, percarbonates, fluorine or hydrofluoric acid, sulfite, or the like, as primary bleaching agents at selected acid or alkaline pH levels and at temperatures ranging from about room temperature up to temperatures well in excess of the boiling point of water at atmospheric pressure. To enhance the effectiveness of these bleaching process or to achieve certain economics or advantages, typical processes employ additives in the bleach baths as buffers, stabilizers wetting agents, chelating agents, and the like, most of which additives are very well known to the art. However, experience has shown that for all the good that such additives are alleged to do, they also do quite a bit of harm. Thus, even the most innocuous of additives have been found to cause violent foaming, weaken the cloth, deplete chemical bleaching agent at unacceptably rapid rates, cause motes, form deposits on the cloth or bleach equipment, be environmentally harmful, or create various other commercially unacceptable conditions.
One of the oldest techniques for enhancing the bleaching process is to pretreat the goods prior to actual bleaching. Depending, of course, upon the precise nature of the goods, pretreatments may be intended to serve specific identifiable purposes. However, as a general rule, the pretreatments serve to remove fats, waxes, starches and other non-cellulosic materials from the cotton goods. Often the pretreatments have been denominated as "desizing" or "scouring"; however, those names have come to be loosely employed and meaningless in the art unless associated with some specific purpose and pretreatment process which sets forth and identifies the chemicals and steps employed, the parameters such as pH, temperature, pressure and concentration, and the pretreatment time. Historically, the most frequently used pretreatments were the severe caustic or lime boils in which the goods were boiled at 15-35 psi in caustic or calcium soda for 4-12 hours. A more modern counterpart of the caustic scours is one in which the goods are impregnated with caustic soda and then steamed at 100.degree. C. for 30-60 minutes. Acid pretreatments are also historically well known, for example, steeping the goods in sulfuric or phosphoric acid for 3-4 hours at room or higher temperature. Other popular pretreatments include immersing the goods in wetting agents, such as turkey red oil or sulfonates, or in enzymes. Steaming the goods prior to bleaching, either as a separate step, or, more commonly, in association with an alkaline bath impregnation or pretreatment, appears to have been a popular and widely used pretreatment procedure. Notwithstanding that these pretreatments may indeed have accomplished their intended objective, such as decreasing bleaching time or enhancing bleached goods whiteness levels, they typically have been and are extraordinarily wasteful of chemicals, and almost without exception have required high temperatures or application for extended periods of time. Thus, although the prior art pretreatments seem to produce some useful results, in fact there are very few net economies associated with any of them. Moreover, subjecting cellulosic goods for extended time periods to highly acidic or highly caustic solutions, or to prolonged high temperature steam treatment, cannot help but weaken the fibers and have an adverse effect upon the tear strength of the goods which ultimately emerge from the bleaching process.